Pointing to unexpected increases in demand for high-octane unleaded gasoline, several oil companies have warned government regulators in recent days of possible future spot shortages unless current federal regulations are changed.

But government and industry officials said unequivocally that future shortages, which could come over the next two years, are not likely to be as severe as those that occurred in 1973-74, nor are motorists likely to face the long lines at gasoline stations that marked that period.

Fear of future shortages was touched off Thursday when Shell Oil Co. announced that it would begin rationing gasoline supplies to wholesale and retail dealers, limiting them to 75 percent of their usual deliveries.

The latest contact between the oil industry and the government came yesterday, when officials of Texaco told Douglas Costle, administrator of the Environmental Protection Agency, that the company may within the next two years have to resort to a rationing program similar to the one announced by Shell.

The Texaco meeting came after similar sessions between EPA officials and representatives of Sun Oil Co. and Exxon. The meetings are part of an effort by the industry to persuade the government to ease up on its program to phase out leaded gasoline for environmental reasons.

Industry officials blame the potential shortages on a quagmire of federal regulations, and some government regulators agree. "Anyone who wants can look at today's controls and see that they are in shambles," John F. O'Leary, deputy energy secretary, said yesterday. He said the oil industry is caught in a squeeze because of conflicting pricing, mileage efficiency and environmental regulations.

The Energy Department has attempted several times in recent years to deregulate the price of gasoline at the pump, but the EPA has joined with several consumer groups in a successful legal effort to block decontrol.

Oil companies, however, have for the most part planned their production schedules on the assumption that gasoline prices would be decontrolled, and that the public would pay the extra price necessary to shift production to more and more high octane lead-free gasoline.

Frank Bradley, Standard Oil of California (Socal) executive, said that in the oil industry's view, "All of the regulations from both DOE and EPA have contributed to this problem." Socal, a California-based major oil company, has not experienced any shortages of unleaded gasoline.

Unleaded gasoline. required under EPA regulations, now accounts for about 33 per cent of domestic gasoline consumption. There are no shortages of leaded gasoline, used in older autos.

For its part, however, EPA has sought to reduce lead emissions in gasoline. Worried about reports that thousands of motorists have been switching back to much cheaper leaded gasoline despite the fact that their autos are equipped to handle the unleaded, the EPA has called for an environmental impact statement to address the fuel-switching problem.

Consumer and environmental groups claim that, under deregulation, the price difference between unleaded and leaded will grow even more than the estimated 4-cent-a-gallon spread that now exists. If so, they claim, more motorists will switch to leaded, and consequently add significantly to pollution problems.

The regulatory skein that confronts the industry, government and the bewildered motorist, who is confused about spot shortages of gasoline while the world oil glut lingers on, is the legacy of Congress' and the executive's shifting goals.

The story began in 1973, when Congress enacted the Emergency Petroleum Allocation Act, which continued a system of supply and price controls, which, among other things, prevents refiners from passing on the costs of their equipment to produce high octane, unleaded gas. Most consumer groups want to continue these regulations, which hold down gasoline prices.

Later in 1973, EPA published regulations mandating a sharp reduction in lead additives in gasoline -- used by refiners to boost octane -- to.5 grams per gallon by 1979. Both Detroit and the oil industry have resisted these rules, which have strong support of environmentalists.

To meet the lead standard goals mandated by EPA and the Clean Air Act, however, car manufacturers began producing cars with lower mileage ratings.

In 1975, however, Congress and the Ford administration joined forces to pass a comprehensive energy bill, the Energy Policy and Conservation Act, which, among other things, continued price controls on some oil and set stiff mileage efficiency standards for new autos. These new standards are to rise to 27 1/2 miles per gallon by the mid-1980s.

That same year, Detroit also produced its first models with catalytic converters, which, if used with leaded gasoline, are made ineffective.

Ford's last major energy decision before leaving office was to submit a gasoline decontrol proposal to Congress. When President Carter took office, he pulled it back, saying he wanted to study the issue.

In April 1977, Carter said he would not press for gasoline decontrol.

But most oil companies expected Carter to send a gasoline decontrol proposal to Congress duiring the fall of 1977. Energy Secretary James R. Schlesinger Jr., however, held it back for fear that it would hinder passage of the Carter energy bill.

For more than a year the conditions for spot gasoline shortages have been building up while Congress and the Energy Department put off ending the regulatory snarl. In the meantime, the industry, in search of higher profits, has held off producing more unleaded gas, and the EPA has not backed down on its regulations.